The overall goal of this grant is to study CDC6 gene function and its cell cycle regulation. Early genetic studies revealed that yeast temperature-sensitive mutant, cdc6, is defective in the late G1 and S phase boundary, and that its function may be, either directly or indirectly, involved in DNA replication. We have previously isolated and characterized the CDC6 gene. We have also demonstrated that CDC6 mRNA and nuclear entry of CDC6 protein are cell cycle dependent. In this submission period, we have found that the GTS::CDC6 fusion protein has a DNA-dependent ATPase activity. In this proposal, we will use a combined molecular genetic and biochemical approach to investigate the biological function(s) of CDC6 and its cell cycle regulation. Initially, we will identify the cellular components which interact with CDC6 gene products. In order to accomplish this, we will isolate and characterize the suppressor(s) to establish a linked pathway between CDC6 and other related genes in the cell cycle progression. We will also use a molecular genetic approach, the 2-hybrid system, to identify the protein component(s) which interact with CDC6 in vivo. The results will complement our genetic studies, providing additional information to establish a pathway of cell cycle progression. Secondly, we will explore the gene function(s) of CDC6 via the biochemical approach. We will characterize the purified CDC6 protein by investigating its nucleotide-binding, ATPase, and/or GTPase activities. Based on the sequence motif, we will also detect possible helicase activity. Point-mutated cdc6 proteins will also be purified to complement the above biochemical studies. It is anticipated that the results will shed light on the biological function(s) of CDC6. We expect that the investigation of the yeast CDC6 gene and its suppressors will lead to an understanding of the functions of corresponding genes in other eukaryotes, and may provide us with useful information to disclose the profound control mechanism of the cell division cycle.